A.T. Robertson Commentary John 14:20

"In that day ye shall know that I am in my Father, and ye in me, and I in you." — John 14:20 (ASV)

In that day (εν εκεινη τη ημερα). The New Dispensation of the Holy Spirit, beginning with Christ's Resurrection and the Coming of the Holy Spirit at pentecost.

Shall know (γνωσεσθε). Future middle of γινωσκω. Chapters 1 to 3 of Acts bear eloquent witness to these words.